Make lib/jake.js the package main file.

Previously if you tried o require Jake as part of other tool
`require('jake')` would fail. This commit sets lib/jake.js as
the main entry point for this package so other tools can take
advantage of using jake.

package.json

@@ -12,7 +12,7 @@
   "bin": {
     "jake": "./bin/cli.js"
   },
-  "main": "./lib",
+  "main": "./lib/jake.js",
   "repository": {
     "type": "git",
     "url": "git://github.com/mde/jake.git"
@@ -23,4 +23,4 @@
   "engines": {
     "node": "*"
   }
-}
+}